Vegetation on and adjacent to mesas in the Nama-Karoo, South Africa : characteristics and comparisons

Thesis (MSc)--University of Stellenbosch, 2002.

ENGLISH ABSTRACT: Arid lands comprise some 47.2 % of the world's land surface. Between 32 % and 35 % of these drylands are subject to some form of desertification and land degradation. South Africa is an arid country with water being a major limiting resource. The Nama-Karoo biome is an extensive tract of semi-arid vegetation comprising some 22.7 % of South Africa, characterised by low rainfall and high temperatures. Grazing is the most common form of land use in the country, and particularly in this biome. Aridity and grazing are two factors that make large areas of South Africa (including the Nama-karoo) susceptible to land degradation. The Nama-Karoo biome has been neglected in past research and is an understudied part of South Africa's vegetation. This study is a component of a bigger umbrella project entitled "Restoration of degraded Nama-karoo rangelands: the role of conservation islands". The aim of this project was to assess the role played by isolated hills and mountains (mesas, inselbergs, etc.) in conserving the remnant biological diversity of the Nama-karoo rangelands of Namibia and South Africa. Vegetation composition, seed banks, seedling recruitment, pollination and grazing patterns and intensity on and off isolated mesas in the eastern parts of the Nama-karoo were investigated in the South African component of this project. I report on patterns of 1) plant species diversity and 2) plant communities across the Middelburg District, Eastern Cape, South Africa. Three isolated mesas (Tafelberg, Folminkskop and Buffelskop) and their surrounding plains were selected for this study. Permanent plots were established in broad transects extending from the plains to southeastern slopes, plateaux, north-western slopes and north-western plains of each mesa. Plots were 25 m2 in size, and ten 1 m2 blocks within the 25 m2 plot were randomly selected and sampled. Species composition was recorded and cover values determined for all plants in every subplot. In the eastern Nama-karoo mesas were not found to be higher in Alpha Diversity than their surrounding plains. Plains habitats were mostly equally as diverse as mesa habitats, with some plains habitats being higher in diversity than mesa habitats. Greater differences between mesas and their surroundings were found in a parallel study in the northern parts of the Nama-karoo in Namibia, indicating that mesas are more distinct islands of diversity with an increase in aridity. In the Middelburg District, a greater presence of woody phanerophytes occurred on mesa habitats in comparison with plains habitats, a possible result of the more moist microhabitats on mesas. Cooler, moister conditions on the south-eastern slopes of the mesas led to higher Alpha Diversity there in comparison with the warmer and drier north-western slopes, a phenomenon commonly found in arid areas. In terms of Alpha Diversity, the largest mesa sampled, Tafelberg (450 m above surroundings), was not significantly more diverse than the two smaller mesas (Folminkskop and Buffelskop both being 200 m above surroundings). Tafelberg and Buffelskop were equally high in Beta Diversity, with Folminkskop being much less diverse. Increased Beta Diversity for Tafelberg was explained by the increased size of the mesa (providing a more diverse microhabitat) relative to Folminkskop, which has the same shape and geology but is much smaller in size. High Beta Diversity values for Buffelskop were explained by the presence of degraded communities on the northwestern slopes and plains, while the south-eastern slopes were some of the most diverse habitats sampled in the landscape. Mesas and plains shared few species. Proportionally fewer species were shared between the plateau and slopes of the larger Tafelberg in comparison with the smaller two mesas. Isolation of the plateau could possibly become more distinct with an increase in size of the mesa. However, Tafelberg had more species in common with the surrounding plains than did the smaller mesas. It is suggested that bigger mesas support a more diverse array of microhabitats enabling plains species to occupy selective sites on the slopes of the mesa. Average Shannon-Wiener Alpha Diversity Indices for Middelburg were higher than that of the Nama-karoo in general. This could be explained by the higher precipitation in the eastern parts of the Nama-karoo relative to the rest. Diversity in Middelburg compared favourably with that in other arid lands of North America and Australia and was found to be similar in patterns and determinants of diversity compared to arid lands such as those of the New World. Regarding plant community composition, mesas were found to be distinctly different from their surrounding plains, with no shared communities between mesas and plains. The distribution of communities across the landscape was mainly attributed to a soilmoisture gradient. Mesas, compared to plains, have shallower, rockier soils coupled with very little run-off and naturally higher precipitation due to elevation. This results in a more mesic habitat. Plains, compared with mesas, have little rock cover, high percentages of bare soil and higher run-off rates than mesas, resulting in a more xeric habitat. Habitat differences such as these probably existed before the impact of domestic livestock but overgrazing has probably exacerbated the differences. The dominance structure of plant communities on the plains has probably changed in favour of toxic, spinescent or unpalatable plant species due to selective grazing by livestock. For the two dolerite-capped mesas (Tafelberg and Folminkskop) aspect and the expected cooler, moister conditions on south-eastern slopes as factors determining community composition were overridden by soil type and associated nutrient status. On Buffelskop (sandstone mesa), aspect and slope overrode soil type and associated nutrient status as determinants of community compositions. The potential to use mesas as a source of seeds and propagules to restore degraded plains habitats is low. Approximately 28 % of species were shared between mesas and their surroundings, and not all of these species could be regarded as good colonisers. Generalist, palatable species occurring across the landscape, such as Eragrostis obtusa, Felicia muricata, F. ovata, Fingerhuthia africana, Jamesbrittenia tysonii, Limeum aethiopicum, Pentzia incana and Selago albida could have some potential for future restoration attempts. Mesas are distinct in composition and can be regarded as islands of one vegetation type (mesas) in a sea of a different vegetation type (plains). Mesa habitats are not generally used for grazing by livestock, due to their general inaccessibility, and are not threatened by development. Plant communities in the Middelburg area were very similar In composition to communities identified in other parts of the Nama-karoo. Results from this study suggest that the plains are at present more degraded than the mesa habitats, but it is impossible to conclude whether or not these rangelands have stabilised given current stocking rates and climatic conditions. The Braun-Blanquet classification system proved an effective method to describe plant communities in the semi-arid Nama-karoo. All the plots used during the duration of the study were marked by metal stakes and are therefore of a permanent nature, so that they can be resampled in future. This might possibly shed some light on questions related to resilience, stability and degradation of the karoo. Restoring rangeland in the future is an important option, however, few farmers would, in the short term, be able to afford costly restoration techniques. It is recommended that farmers inspect the condition of their veld on a regular basis, and adjust stocking rates accordingly. Veld should be rested on a regular basis, and assessment should precede the movement of stock to a camp. Most farmers plough denuded areas of veld to encourage restoration, and it is recommended that this practice be combined with reseeding, followed by rest during and shortly after germination of the seed.

AFRIKAANSE OPSOMMING: Ariede areas beslaan tans 47 % van die aarde se land oppervlak. Tussen 32 % en 35 % van hierdie areas is onderworpe aan een of ander vorm van land degredasie. Die Nama- Karoo bioom is 'n ekstensiewe semi-ariede area wat naastenby 47 % van Suid-Afrika beslaan. Die karoo word gekenmerk deur hoë temperature en lae reënval. Suid-Afrika is 'n ariede land en water is 'n beperkende faktor. Weiding is die algemeenste boerderypraktyk, en meer so in ariede areas soos bv. in die karoo. Die droë klimaat en hoë voorkoms van weiding as boerderypraktyk maak ekstensiewe areas (insluitende die karoo) van Suid-Afrika vatbaar vir land degredasie. Hierdie studie is deel van 'n breër projek genaamd: "Restorasie van gedegradeerde Nama-karoo veld: Die rol van bewaringseilande". Die doel van hierdie projek was om die rol te bepaal van ge-isoleerde koppies en berge (mesas, inselberge, ens.) in die bewaring van biologiese diversiteit in die Nama-karoo van Suid-Afrika en Namibië. Plantegroeisamestelling, saadbanke, saailing oorlewing, bestuiwing en weidingsintensiteitlpatrone op koppies en hulle omringende vlaktes in die oostelike dele van die Nama-karoo is ondersoek in die Suid-Afrikaanse komponent van die studie. Ek rapporteer oor patrone van 1) plant diversiteit en 2) plantegroeigemeenskappe in die Middelburg Distrik, Oos-Kaap, Suid-Afrika. Drie ge-isoleerde koppies (Tafelberg, Folminkskop en Buffelskop) en hulle omliggende vlaktes is geselekteeer vir die doel van die studie. Permanente persele is uitgelê in 'n breë lyn vanaf die suid-oostelike vlaktes na die suid-oostelike hang, oor die plato's, noord-westelike hange en noord-westelike vlaktes vir al drie koppies. Persele was 25 m2 in grootte, waarvan tien 1 m2 sub-persele geselekteer is binne die groter 25 m2 blok. Spesies samestelling en bedekkingswaardes is bepaal vir alle plante in al die sub-persele. Mesas was nie hoër in Alpha Diversiteit as hulle omringende vlaktes nie. Vlaktes was meestal net so hoog in diversiteit as die mesas, en somtyds selfs hoër. Groter verskille in diversiteit is gevind in 'n paralelle studie in die noordelike dele van die Nama-karoo in Namibië. Dit dui aan dat mesas moontlik meer definitiewe eilande van diversiteit is in droër gebiede. Meer houtagtige fanerofiete het voorgekom op die mesas in vergelyking met die omliggende vlaktes, 'n moontlike gevolg van 'n natter habitat op die mesas. Suid-oostelike hange was hoër in alfa-diversiteit in vergelyking met noord-westelike hange. Dit is toegeskryf aan die koeler, natter mikrohabitat van die suid-oostelike hange. Die groter mesa (Tafelberg - 450 m bo die vlakte), was nie meer divers as die twee kleiner mesas nie (Folminkskop en Buffelskop is sowat 200 m bo die vlakte). Tafelberg en Buffelskop was ewe hoog in Beta Diversiteit, terwyl Folminkskop heelwat laer was. Hoër Beta Diversiteit vir Tafelberg kan verklaar word deur die groter oppervlak en hoogte van die mesa (voorsien 'n meer diverse mikrohabitat) in vergelyking met Folminkskop. Folminkskop het dieselfde vorm en geologiese geskiedenis as Tafelberg, maar is veel kleiner. Buffelskop was hoog in Beta Diversiteit as gevolg van 'n gedegradeerde gemeenskap op die noord-westelike hang en vlaktes, terwyl die suidoostelike hang baie hoog was in diversiteit. Min spesies kom op beide mesas en vlaktes voor. Minder spesies is gedeel deur die plato en hange van Tafelberg in vergelyking met die plato's en hange van Folminkskop en Buffelskop. Isolasie van die plato kan moontlik hoër wees in groter mesas. Tafelberg (slegs mesa) het meer spesies in gemeen met sy omliggende vlakte as die ander twee mesas. Groter mesas soos Tafelberg kan moontlik 'n meer diverse spektrum van mikrohabitatte bevat, wat sekere vlakte spesies in staat stel om te oorleef op mesa hange. Gemiddelde alfa diversiteit vir die Middelburg Distrik was hoër as die van die res van die karoo. Dit kan verklaar word deur die hoër reënval in die oostelike dele van die karoo relatief tot die meer westelike dele. Diversiteit in Middelburg vergelyk goed met die van ander ariede lande in Noord-Amerika en Australie, terwyl patrone en faktore wat diversiteit bepaal soortgelyk was aan die van gemeenskappe in ariede lande van die Nuwe Wêreld. Mesas en vlaktes het drasties verskil in die samestelling van hulle onderskeie plantegroeigemeenskappe, alhoewel hulle soortgelyk was in diversiteit. Geen plantegroeigemeenskappe het voorgekom op beide mesas en vlaktes nie (gedeelde plantegroeigemeenskappe). Die verspreiding van plantegroeigemeenskappe oor die landskap was toegeskryf aan 'n grond-water gradient. Mesas het, oor die algemeen, vlakker, meer rotsagtige grond en 'n hoër reënval met minder afloop as vlaktes. Vlaktes het minder rotse, 'n hoër persentasies kaal grond en vinniger afloop tempo's na reën as mesas. Hierdie faktore het tot gevolg dat mesas 'n natter habitat verteenwoordig in vergelyking met die meer ariede vlaktes. Habitatsverskille soos dié het heel moontlik klaar bestaan voor die aankoms van vee, maar oorbeweiding het die verskille in habitat tussen mesas en vlaktes groter gemaak. Selektiewe beweiding deur vee het waarskynlik tot gevolg gehad dat plante wat giftig, doringagtig of onsmaaklik is, toegeneem het in plantegroeigemeenskappe, ten koste van meer smaaklike spesies. Grondtipe en grondsamestelling het aspek en verwagte koeler kondisies op die suidoostelike hange onderdruk as bepalende faktore vir plantegroeigemeenskap samestelling op Tafelberg en Folminkskop (beide bedek met 'n doleriet laag). Aspek en steilte van die hange het grondtipe en grondsamestelling onderdruk as bepalende faktore vir plantegroeigemeenskappe op Buffelskop. Die potensiaal om mesas te gebruik as bronne van saad vir die rehabilitasie van die vlaktes was laag. Mesas en hulle omliggende vlaktes het naastenby 28 % van spesies in gemeen gehad, maar nie al hierdie spesies was goeie koloniseerders nie. Sekere smaaklike plante wat op beide mesas en vlaktes voorgekom het, kan potensiaal hê vir toekomstige restorasie doeleindes, bv. Eragrostis obtusa, Felicia muricata, F. ovata, Fingerhuthia Africana, Jamesbrittenia tysonii, Limeum aethiopicum, Pentzia incana en Selago albida. Mesas is uniek in samestelling en kan beskou word as een plantegroeitipe in 'n see van 'n ander tipe (vlaktes). Mesas word nie oor die algemeen bewei nie (as gevolg van hulle steil hange en bergagtigheid) en word gevolglik nie bedreig deur huidige ontwikkeling nie. Plantegroeigemeenskappe in Middelburg is soortgelyk aan die van ander dele van die Nama-Karoo bioom. Resultate van die studie dui aan dat die vlaktes huidiglik meer gedegradeer is as mesas, maar dit is onbekend of die veld gestabiliseer het of steeds besig is om verder te degradeer in huidige weidings- en klimaatskondisies. Die Braun-Blanquet klassifikasiemetode IS effektief vir die beskrywing van plantegroeigemeenskappe in ariede areas. Al die persele in die studie area is gemerk met metaal paaltjies en is dus permanent. Dit maak dit moontlik om in die toekoms die persele weer te ondersoek. 'n Langtermyn datastel kan moontlik lig werp op die stabiliteit en degredasie van karoo veld. Restorasie van veld is 'n moontlikheid vir die toekoms, maar min boere kan duur restorasie metodes bekostig in die kort termyn. Boere moet hulle lande gereeld ondersoek en drakrag aanpas by die kondisie van die veld. Baie boere ploeg kaal kolle in die veld op 'n gereelde basis, en daar word voorgestel dat die praktyk gekombineer word met die saai van geskikte saad, gevolg deur 'n rusperiode tydens en na ontkieming daarvan.